Re: Relativistic transit through another solar system



Mike Williams wrote: 
Wasn't it  who wrote:

We want to fire off a probe to Alpha Centauri.  Assume for argument's
sake that we can get it up to 0.5c -- laser cannon, whatever -- but
have no means of braking it at the other hand.  So it will transit the
Centauri system at half light speed.

This gives it just over a day insystem, after just under nine years
travel.  "In system" here being broadly defined as 40 au, orbit of
Pluto equivalent. There's not much of anything "behind" the Centauri
system, so the probe would be a dead loss after its transit, but never
mind that now.

Couple of thoughts.

1)  Hitting anything much bigger than a molecule will finish the probe
off.  At 0.5c, a grain of dust just big enough to see will hit like a
hand grenade.

Based on what we know about the interplanetary medium in our solar
system, what are the odds of such an encounter?  Is there anything we
might do to improve them?



There are something like 10^22 dust particles with sizes in the range
5-100 micrometres in our solar system, which would give an average
density of one particle per 10^17 cubic metres. That would suggest that
a spacecraft would have a 0.13% chance of encountering such a particle
for each square metre of the probe's cross section.


I get .013%


However, the density of the dust particles varies considerably. There's
a lot more dust as you get closer to the Sun, and there are considerable
concentrations associated with the orbits of comets. I don't have
figures for the variations.

The trajectory's angle with the ecliptic plane is important. If it's perpendicular the chances are much smaller, coplanar the odds are higher. A pass as close as to Mercury in a coplanar path guarantees two passes through each asteroid belt the system has.

--
Hop David
http://clowder.net/hop/index.html

.

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